The present invention relates to a coordinate input device, and more particularly to a coordinate input device for generating a coordinate data for control of a cursor position or the like on a screen by rotating a ball in any direction.
Conventionally, a coordinate input device is used to control a cursor position, a figure coordinate position, etc. on a screen of a display such as a CRT by rotating in any direction a ball partially projecting through an opening of a housing. In general, such a coordinate input device includes a housing having an opening, a ball received in the housing so as to be rotatable in any direction, the ball partially projecting through the opening, a pair of rotating shafts arranged orthogonally to each other in the housing and adapted to be rotated in association with rotation of the ball, a rotation detector for detecting a quantity of rotation of each of the rotating shafts, and a printed board mounted on the housing, the rotation detector being mounted on the printed board. In general, an annular retainer for retaining the ball in the housing is detachably mounted to the opening of the housing, and springs for biasing the rotating shafts against a surface of the ball is provided in the housing. There is a case that the surface of the ball is stained or damaged in the use of such a coordinate input device. In such a case, it is necessary to remove the retainer and take the ball out of the housing through the opening for the purpose of cleaning or replacement of the ball. Accordingly, the opening for allowing pass of the ball is normally formed on the side of the housing opposite to a surface for mounting the printed board. On the other hand, ball supporting members for supporting the ball, the rotating shafts, the spring, etc. normally need not be subjected to the maintenance such as cleaning and replacement, so that they are preferably not allowed to be mounted and demounted from the opening side of the housing.
Further, in recent years, there is a tendency that such a ball type coordinate input device is directly installed in a keyboard of a computer, particularly a small-sized computer such as a laptop personal computer, notebook-size personal computer, or handy personal computer, or is installed in a small-sized case of a pen type. Accordingly, the ball to be employed in the coordinate input device of this type tends to be increasingly reduced in diameter, and the supporting members for supporting the ball tend to also be increasingly reduced in size. As a result, a work of mounting the supporting members into the housing becomes difficult more and more.
In the prior art, the rotating shafts, a part of the ball supporting members for supporting the ball, etc. are mounted from the ball receiving side of the housing, that is, from the side opposite to the printed board mounting surface of the housing, so that there is a possibility that the rotating shafts, the ball supporting members, etc. unduly come out of the housing. Further, the printed board is mounted on the housing from one side of the housing, while a part of the ball supporting members, the rotating shafts, etc. must be mounted into the housing from the other side opposite to the printed board mounting surface. Accordingly, an assembly operation of the coordinate input device becomes troublesome to cause an increase in cost.
Meanwhile, there is a tendency that a spherical member is used as the ball supporting member in accordance with the tendency of downsizing of the ball supporting member. FIG. 12 is a plan view of an essential part of a mounting structure of such a spherical supporting member in a conventional coordinate input device. As shown in FIG. 12, a housing 1 is formed with a groove 3 for receiving a part of a spherical supporting member 2. The spherical supporting member 2 is fixed in the groove 3 by caulking the opposite sides of the groove 3 as depicted by reference numerals 4, or it is fixed in the groove 3 by adhesive.
However, in the case of the fixing method by caulking, the caulked portion is apt to be loosened by aged deterioration, causing a possibility that the supporting member 2 comes out of the groove 3. Further, an expensive caulking apparatus is required, and a caulking step is additionally required in the assembling of the coordinate input device, causing a reduction in assembly operability.
In contrast, in the case of the fixing method by adhesive, it is difficult to apply a suitable amount of adhesive to a suitable portion of the small spherical supporting member 2, and there is a possibility that the adhesive overflowed from the groove 3 comes into contact with the ball to damage the slidability of the ball. Further, an adequate time for drying and hardening the adhesive is required to cause a reduction in assembly operability of the coordinate input device.
In a conventional coordinate input device wherein the ball is rotatably supported at three points by two rotating shafts and a single supporting member, a depression force applied to the ball is applied to the rotating shafts. Therefore, when an excessive depression force is applied to the ball, the rotating shafts are apt to be deformed, so that they cannot follow the rotation of the ball to interfere with detection of the rotation. If a diameter of each rotating shaft is enlarged, so as to prevent the deformation of the rotating shafts, a compact construction of the coordinate input device as a whole becomes difficult to achieve.
Further, in a conventional coordinate input device wherein the ball is rotatably supported at three points by three fixed supporting members provided in the housing, the above-mentioned problem can be eliminated. However, each rotating shaft is biased against the ball by means of springs in the vicinity of opposite ends of the rotating shaft, so that possible variation in biasing force of the springs brings about the difficulty of retention of each rotating shaft in a fixed position. As a result, there occurs slippage in position of a rotating plate mounted on each rotating shaft relative to a rotation detector for detecting a quantity of rotation of the rotating plate, causing a reduction in detection accuracy of the rotation detector.